First order transition films for magnetic recording and method of forming

ABSTRACT

Thin films of iron-rhodium exhibiting a broadly hysteretic first order transition between the ferromagnetic and antiferromagnetic states are produced by sequentially depositing iron and rhodium films upon a refractory substrate at a pressure in the range of 1 X 10 6 torr, annealing the structure in a vacuum of 1 X 10 6 torr at a temperature of approximately 700* C. for 1 hour to produce a complete diffusion of the iron and rhodium layers, and subsequently subjecting the diffused layers to a second anneal in an atmosphere greater than 10 parts per million oxygen in a thermal cycle that includes slowly heating the structure to 400* C., maintaining the 400* C. for approximately 10 minutes and slowly cooling to room temperature. Films thus formed are advantageously employed in the recording of digital information by electron beam heating individual regions through a first order transition to the ferromagnetic state whereupon the regions are permitted to cool to a biasing temperature slightly higher than the temperature of transition back to an antiferromagnetic state. A magnetic field then is applied to the entire film to magnetize only those regions of the film in the ferromagnetic state and readout of the recorded information can be achieved by conventional electron beam microscopy. The ferromagnetism of the film subsequently can be erased by cooling the film below the transition temperature to the antiferromagnetic state or by the application of a strain to the film.

United States Patent [72] Inventor James M. Lommel Schenectady, N.Y.

21 Appl. No. 776,619

[22] Filed Nov. 18, 1968 [45] Patented Sept. 21, 1971 [73] AssigneeGeneral Electric Company [54] FIRST ORDER TRANSITION FILMS FOR MAGNETICRECORDING AND METHOD OF 340/174 TF; 148/3l.55, 108

[56] References Cited UNITED STATES PATENTS 3,140,941 7/1964 Walter75/172 X 3,140,942 7/1964 Walter. 75/172 X 3,160,576 12/1964 Eckert.148/108 X 3,415,695 12/1968 Kouvel 148/31.55 X

Primary Examiner-L. Dewayne Rutledge Assistant Examirggr G. K. WhiteAttorneys-Richard R. Brainard, Paul A. Frank, John J.

Kissane, Frank L. Neuhauser, Oscar B. Waddell and Melvin M. GoldenbergABSTRACT: Thin films of iron-rhodium exhibiting a broadly hystereticfirst order transition between the ferromagnetic and antiferromagneticstates are produced by sequentially depositing iron and rhodium filmsupon a refractory substrate at a pressure in the range of 1X10 torr,annealing the structure in a vacuum of 1X10 torr at a temperature ofapproximately 700 C. for 1 hour to produce a complete diffusion of theiron and rhodium layers, and subsequently subjecting the diffused layersto a second anneal in an atmosphere greater than 10 parts per millionoxygen in a thermal cycle that includes slowly heating the structure to400 C., maintaining the 400 C. for approximately 10 minutes and slowlycooling to room temperature. Films thus formed are advantageouslyemployed in the recording of digital information by electron beamheating individual regions through a first order transition to theferromagnetic state whereupon the regions are permitted to cool to abiasing temperature slightly higher than the temperature of transitionback to an antiferromagnetic state. A magnetic field then is applied tothe entire: film to magnetize only those regions of the film in theferromagnetic state and readout of the recorded information can beachieved by conventional electron beam microscopy. The ferromagnetism ofthe film subsequently can be erased by cooling the film below thetransition temperature to the antiferromagnetic state or by theapplication of a strain to the film.

PATENTEU 8832 I97! SHEET 1 OF 3 FIG.

ANNEAL AT IO TORR.

TO DTFFUSE THE ANNEAL Fe Rh FILM IN NITROGEN ATMOSPHERE CONTAININGOXYGEN IN Fe AND Rh LAYERS LAYERS QUANTITIES GREATER THAN IO PPM FIG. 2[50- 2 K (9 D v E M 2 Q s z Fe Rh FILM PRIOR TO 5 OXIDATION ANNEAL Z O 2o I J I l l J -2oo -loo 0 I00 200 TEMPERATURE, c

2 g M Fe Rh FILM AFTER 2 OXIDATION ANNEAL m A loo-- 2 2 E tr 2 50- L) Eo l -2oo -loo 0 h I00 200 TEMPERATURE, c

INVENTOR:

JAMES M. LOMMEL,

/-//s ATTORNEY PATENTEU SEPZI l9?! SHEET 3 BF 3 33 MAGNETIZATION SHIFT VUPON RUBBING Fe Rh SURFACE IOO TEMPERATURE, "0

FIG. 8

INITIAL MAGNETIZATION REMANENT MAGNETIZATION H, OERSTEDS C m YEA/TOR.

JAMES M. LOMMEL, bygz l5 ATTORNEY TEMPERATURE,

FIRST ORDER TRANSITION FILMS FOR MAGNETIC RECORDING AND METHOD OFFORMING THE DISCLOSURE This invention relates to iron-rhodium filmshaving a substantially complete first order transition between themagnetic and nonmagnetic states on heating and the methods of formingsuch films. These films are particularly useful in the recording ofdigital information by a unique recording scheme wherein information isstored by the conversion of individual regions to a magnetic staterather than by the relative alignment of the magnetization in selectedregions of completely magnetic films. The invention described herein wasmade in the course of or under a contract or subcontract thereunder withthe Department of the Air Force.

The intermetallic compound iron-rhodium has been a source of scientificcuriosity because of the first order transition exhibited by the bulkmaterial in abruptly transforming from the antiferromagnetic to theferromagnetic states upon heating to a temperature above approximately60 C. Films of the compound heretofore produced by conventional filmforming techniques however, have been characterized by a broad thermalhysteresis (contrary to the narrow thermal hysteresis of approximatelyC. exhibited by the well annealed bulk material) and only a partialtransition to the antiferromagnetic state upon cooling. Such resultshave been reported in the Journal of Applied Physics, Vol. 37, No. 3,1483-1484, March 1, 1966. It is a particular object of this invention toprovide a thin film of iron-rhodium exhibiting a broad thermalhysteresis with a high percentage transition between the ferromagneticand antiferromagnetic states and the method of forming of such a film.

The first order transition characteristics of the film also permit therecording of digital information by novel techniques. For example, priorto this time digital information has been recorded upon magneticmaterials by an orientation of the magnetic domains at selected regionsin a chosen direction. I1- lustrative of these prior techniques is Curiepoint writing, wherein information is stored by aligning themagnetization of regions selectively heated above the Curie temperatureof the magnetic film and cooling the film in the presence of an aligningmagnetic field. Information then can be readout utilizing themagneto-optic Kerr or Faraday effect. Similarly, in compensation pointwriting, heating of selected regions of a gadolium iron garnet film by alaser or electron beam decreases the coercive force of the heatedregions to permit alignment of the magnetization of the heated regionsutilizing an externally applied field.

It is an additional object of this invention to provide a novel digitalrecording technique wherein information is thermally recorded by aconversion of selected regions of a recording film between the magneticand the nonmagnetic states in a first order transition.

A further object of this invention resides in providing a novelrecording medium for the storage of digital information in accordancewith the recording techniques of this invention.

It is still another object of this invention to provide a recordingmedium wherein the ferromagnetism can be mechanically erased.

These and other objects of this invention generally can be achievedutilizing a thin, e.g. less than 1 mil thick, iron-rhodium film having acomposition range between 50-65 atom percent rhodium and characterizedby a first order transition between the ferromagnetic andantiferromagnetic states in excess of 50 percent of the film whentemperature cycled through the thermal hysteresis loop of the film. Toprovide a suitable thermal tolerance for recording, the film preferablyhas a thermal hysteresis loop having a width between 10 C. and 200 C. atthe mean magnetization of the film.

Iron-rhodium films having these characteristics generally can be formedby positioning a conventional iron-rhodium film, e.g. a 50-65 atompercent rhodium film characterized by a thermal hysteresis in excess of50 C. and a transition of less than 50 percent of the film between theferromagnetic and antiferromagnetic state upon thermal cycling, in anatmosphere containing oxygen in a quantity greater than 10 parts permillion and annealing the film in the partial oxygen atmosphere toincrease the portion of the film undergoing transition to an amount inexcess of 50 percent of the films. For a broad thermal hysteresis in thefilm, the oxidation anneal preferably is conducted for a period between5 minutes and 4 hours at a temperature between C. and 800 C. and theoxygen content of the atmosphere wherein the anneal is conducted is lessthan approximately 1,000 parts per million oxygen.

Information is recorded in digital form upon the medium by selectivelyheating regions of the film in the nonmagnetic state to a temperaturewhereat a first order transition to the magnetic state is effected andapplying a magnetic field to the film to magnetize the regions of thefilm converted to the magnetic state. Thus the digital information isstored by a unique method which comprises the recording of informationof a first magnitude as a magnetic region in a film of homogeneouscomposition and recording information of a second magnitude as ajuxtaposed nonmagnetic region in the film. The location of themagnetized regions of the film then can be detected by conventionalreadout means such as electron beam microsco- The novel featuresbelieved characteristic of the invention are set forth in the appendedclaims. The invention itself, together with further objects andadvantages thereof may best be understood by reference to the followingdescription, taken in connection with the accompanying drawings, inwhich:

FIG. I is a block diagram depicting a technique for the formation offirst order transition iron-rhodium films in ac cordance with thisinvention,

FIG. 2 is a graph depicting the variation of magnetization withtemperature in an iron-rhodium thin film prior to an oxidation anneal,

FIG. 3 is a graph depicting the variation of magnetization withtemperature in an iron-rhodium film subsequent to an oxidation anneal inaccordance with this invention,

FIG. 4 is a graph depicting the variation of magnetization, hysteresiswidth and percent transition of an iron-rhodium film with the oxygencontent of the second annealing atmosphere,

FIG. 5 is an enlarged sectional view of a digital recording mediumemploying the iron-rhodium film of this invention as a recording film,

FIG. 6 is an enlarged pictorial illustration of one method of readoutfrom the film,

FIG. 7 is a graph illustrating the effect of stress upon an iron-rhodiumfilm,

FIG. 8 is a graph depicting the magnetic field strength required toproduce a given magnetization in an iron-rhodium film formed inaccordance with this invention, and

FIG. 9 is a graph depicting the variation of magnetization withtemperature at diverse locations along the thermal hysteresis loop ofthe iron-rhodium film.

A preferred method of forming an iron-rhodium film having a broadlyhysteretic first order transition between the ferromagnetic andantiferromagnetic states is depicted generally in FIG. 1 and initiallyincludes the preparation of a conventional iron-rhodium film having abroad thermal hysteresis and an incomplete transition, i.e. a minimummagnetization between one-half and three-quarters the maximum film magnetization, by sequentially vacuum depositing iron and rhodium filmsupon a refractory substrate and annealing the deposited films in avacuum better than 10" torr at a temperature between 400 and 700 todiffuse the layers To obtain a first order complete transition in thealloy film, a subsequent anneal of the film is conducted in a flowingnitrogen atmosphere containing oxygen in quantities greater than 10parts per million.

The sequential vacuum deposition of the iron and rhodium filmspreferably is accomplished by electron beam heating iron and rhodiumsources positioned upon a water cooled hearth within a vacuum chamberwhich is evacuated to a pressure less than approximately 10 torr toreduce oxidation of the films during the evaporation. Typically, theiron and rhodium are evaporated in a vacuum of approximately 10' torrrange with the sequence of the deposition of the alternate layers notbeing important although preferably the iron film is deposited initiallyand subsequently overlayed with rhodium to inhibit oxidation of the ironduring the subsequent anneal. Similarly, the number of alternatelydeposited layers is riotof significance provided the respectivethicknesses of the layers result in a 50-65 atom percent rhodiumcomposition film when annealed to diffuse the layers. For recordingpurpose wherein a low remanent magnetization is desirable, a highrhodium percentage, e.g. approaching 65 atom percent, is advantageouslyemployed.

The deposition rate employed in forming the alternate iron and rhodiumlayers generally is not critical with deposition rates of 4A per secondto 25A per second being suitably employed for the depositions. Thehigher deposition rates are preferred however because of the reducedfilm contamination produced by the more rapid deposition. Duringdeposition of the alternate layers upon the substrate, the substratepreferably is heated, e.g. to 300 C. to reduce the stress in the rhodiumfilm deposited thereon and increase its adherence to the substrate. Thesubstrate employed for the deposition of the alternate layers thereondesirably is a refractory material, e.g. fuse silica, alumina, siliconor clear sapphire, to inhibit diffusion of impurities into the film fromthe substrate during annealing and to withstand the elevatedtemperatures of the diffusion anneal, e.g. up to 700 C.

In general, the iron and rhodium sources employed in the vacuumevaporation should be of high purity although minor amounts of someimpurities can be tolerated and often are beneficial to the magneticcharacteristics of iron-rhodium. For example, certain materials, such asmolybdenum, nickel, copper and niobium can completely destroy thetransition characteristics of iron-rhodium if present in quantities inexcess of 2 atom percent. Other materials, however, serve to shift thetransition temperature of the iron-rhodium and can be beneficiallyemployed to position the magnetic hysteresis curve of the alloy at adesired thermal value, i.e. less than 10 atom percent ruthenium, osmium,iridium, and platinum tend to increase the critical transitiontemperature of iron-rhodium while palladium, vanadium, manganese, andgold in quantities of 10 atom percent or less tend to decrease thecritical transition temperature of ironrhodium. When it is desired thatone or more of these impurities be incorporated into the film, theimpurity can be codeposited with the iron or rhodium layers as an alloyor deposited as a separate alternate layer in a thickness suitable forthe desired percentage of impurity in the film. The total thickness ofthe deposited layers however characteristically is less than 1 mil withtypical films having thicknesses between 200 A and 3,000 A.

After deposition of the iron and rhodium films upon the substrate, thestructure is annealed at a temperature between 400 and 700 C. in avacuum below 10 torr for the period required to completely diffuse thelayers together thereby forming the intermetallic iron-rhodium compound.The diffusion anneal preferably is conducted in a very good vacuum, e.g.torr, to inhibit an excessive oxidation of the film deleterious to thefilm transition characteristics while annealing at temperaturessubstantially above 700 C. tends to produce island structures in theiron-rhodium film reducing the adherence of the film upon the substrate.Because a very fine island structure also can reduce lateral heat flowbetween juxtaposed bit sites in the writing of information into thefilm, the size ofa bit site advantageously could be reduced by a hightemperature diffusion anneal.

In general, the period of the diffusion anneal is not critical providedthe evacuation chamber is of a reduced oxygen content, for example, at avacuum of 4X 10 torr in a dynamically pumped vacuum system employing a2-inch oil diffusion pump and a liquid nitrogen cold trap to preventcontamination of the chamber, complete diffusion of the layers and abroadly hysteretic transition in the magnetic properties of the filmwith temperature was obtained when the diffusion anneal was conducted at700 C. for periods between 1 hour and 25 hours. When the oxygen contentof the diffusion anneal chamber is increased, an excessively protracteddifiusion anneal can substantially oxidize the intermetallic compoundformed by the anneal thereby destroying the thermal hysteresis of thefilm. Annealing at 400 C. for periods as short as 1 hour has been foundto produce films exhibiting a magnetic hysteresis upon thermal cyclingand a physical structure identical to that of bulk samples of theintermetallic compound iron-rhodium when examined by X-ray diffraction.

The thermal hysteresis curve of an iron-rhodium film containing 0.56atom fraction rhodium and formed by a diffusion anneal at 690 C. for 1hour at a pressure of 4 10 torr when temperature cycled in the presenceof a 1,000 oersted field is characterized by the curve of FIG. 2, i.e.less than 50 percent of the film undergoes a transition between theferromagnetic and antiferromagnetic states with temperature cycling. Thethermal hysteresis of the film is broad, being in excess of C. at themean magnetization of the film in comparison to well annealed bulkiron-rhodium which characteristically exhibits a thermal hysteresis of10 C. or less.

The fraction of film undergoing transition between the ferromagneticstate and the antiferromagnetic state can be determined approximatelyfrom the formula,

F M 1-04 2 )l M 1 wherein F is the fraction of film undergoingtransition M is the maximum magnetization of the film upon cooling afterheating the film above the transition temperature of the film, and M isthe minimum magnetization of the film upon heating after cooling thefilm below the film transition temperature.

For films which transform completely to the antiferromagnetic state, theminimum magnetization (M on heating is zero and F is equal to 1. It thusmay be stated that the ironrhodium intermetallic compound films formedby the diffusion anneal are characterized by a first order transition inwhich less than 50 percent of the film transforms between theferromagnetic and antiferromagnetic states and in which the thermalhysteresis is in excess of 50 C. at the center of the magnetic thermalhysteresis loop of the film.

After annealing the layered film structure to form a homogeneousiron-rhodium film having the broadly hystcretic incomplete transitioncharacterized by FIG. 2, the iron-rhodium film is again annealed in anatmosphere containing oxygen in quantities greater than 10 parts permillion to produce a film having a substantially complete transitionsuch as is characteristically displayed by the curve of FIG. 3.Preferably the anneal is conducted at a temperature of approximately 400C. with the film being raised from room temperature to 400 C. inincrements of approximately 6 C./min., held at 400 C. for approximately5 minutes and cooled back to room temperature at a rate similar to theheating rate. The atmosphere employed during the anneal preferably isone atmosphere of flowing nitrogen containing oxygen in quantitiesbetween 10 parts per million and 1,000 parts per million as measured bythe oxygen sensor described in US. Pat. application Ser. No. 554,443,and now abandoned filed June 1, 1966 in the name of H. S. Spacil andassigned to the assignee of the present invention. In general, thequantity of oxygen present during the oxidation anneal is dependent uponthe temperatures employed for the anneal with higher temperaturessignificantly reducing the quantity of oxygen required. Similarly, theoxygen content needed in the annealing chamber varies as a function ofthe time employed for the anneal. Annealing for periods in excess of 1hour at 400 C. at a partial pressure above 10 parts per million oxygencan substantially oxidize the iron-rhodium intermetallic compoundthereby destroying the magnetic hysteresis of the film. The annealingdesirably is conducted for periods between 5 minutes and 4 hours attemperatures of 400 C. when the oxygen content of the annealingatmosphere is between 10 and 200 parts per million. in general. theoxygen content, temperature and period of the oxidation anneal areinterdependent and controlled to produce an oxidation of theiron-rhodium film sufficient to ef feet a transition in excess of 50percent of the film without unduly oxidizing the film to reduce thesaturation magnetization below a recordable level. Iron-rhodium filmsgiven a second anneal in one atmosphere nitrogen containing oxygen inconcentrations as high as l parts per million exhibited a nearlycomplete transition between the ferromagnetic and antiferromagneticstates. The thermal hysteresis loop of the film however was narrow, i.e.l0 C. at the mean magnetization, and the transition between theantiferromagnetic and ferromagnetic states was gradual with a transitionin approximately 90 percent of the film requiring an increase intemperature beyond the film critical transition temperatureapproximately twice that required for films annealed in one atmospherenitrogen containing less than l00 parts per million oxygen.

The effect of oxygen during the second anneal upon the magneticproperties of an iron-rhodium film given a second anneal for 10 minutesat 400 C. subsequent to a diffusion anneal at 700 C. for 1 hour in avacuum of 5X10" torr is depicted in FIG. 4. As can be noted from thepercent film transition curve, identified by reference numeral 14,transition in over 80 percent of the iron-rhodium film occurred atoxygen levels slightly above 7 parts per million oxygen in oneatmosphere of flowing nitrogen and the percent of film undergoingtransition increased from an initial value (identified by referencenumeral 15) below 40 percent to a value of 90 percent when annealed for10 minutes at oxygen concentrations above 50 parts per million. Thehysteresis width of the transition at the mean magnetization,illustrated by curve 16, decreased from an original value (identified byreference numeral 17) in excess of 160 C. prior to the second anneal toa value of approximately 80 C. when annealed in a nitrogen atmospherecontaining 40 parts per million oxygen. The maximum magnetization of theironrhodium film (identified by reference numeral 18) remainedessentially constant at approximately l20 emu/gm. notwithstanding thediverse oxygen contents of the second anneal.

ln specifically forming one iron-rhodium film in accordance with thisinvention, a 99.9 percent electrolytic iron source and a 99.9 percentrhodium source were positioned upon a water cooled crucible in aconventional vacuum bell jar having a 4 inch oil diffusion pump and anantimigration liquid nitrogen cold trap. The iron source had beenannealed in dry hydrogen for 1 inch at 900 C. to reduce the oxygencontent of the iron. The vacuum system was evacuated to approximately 8l0 torr and both sources were melted separately by a 2 KW electron gunto reduce outgassing during evaporation. The iron source then waselectron beam evaporated at a pressure of 8X 1 Otorr and deposited at arate of approximately 12 A/sec. upon a clean fused silica substratepositioned 25 cm. from the sources and heated to a temperature of 275300C. After deposition of the iron layer, the rhodium source was evaporatedat a pressure of 4 l0 torr and deposited at ISA/sec. atop the iron filmupon the heated substrate to a thickness sufficient to produce a 0.54atom fraction rhodium film upon subsequent diffusion of the layeredstructure. The total thickness of the films was approximately 550A. Thestructure then was given a diffusion anneal at 690 C. for 13 1 hour at8X10" torr in a dynamically pumped vacuum system. After cooling thestructure to room temperature, the film exhibited a microstructurehaving as a major component the CsCl structure phase typical of bulkFeRh samples with an f.c.c. phase with a lattice parameter about thesame as elemental rhodium. The thermal hysteresis loop of the film in a1,000 oersted field is depicted in FIG. 2, and shows an incompletetransition between the antiferromagnetic and ferromagnetic states with alarge thermal hysteresis at the mean magnetization ofthe film.

The structure then was given a second anneal in a gaseous environment ofone atmosphere of flowing nitrogen containing an oxygen concentrationbetween ppm. and ppm. with the rate of gaseous flow through the systembeing approximately 2 ft./hr. The structure was raised from roomtemperature to 400 C. in approximately 5 minutes, held at 400 C. forabout 10 minutes and cooled to room temperature the same rate. The filmexhibited an approximately 95 percent transition, as portrayed in thethermal hysteresis loop of HO. 3 when temperature cycled in a 1,000oersted field, and a large thermal hysteresis of approximately 80 C. atthe mean magnetization of the film. The transition was sharper uponheating, occurring within approximately 60 C., then on cooling where atemperature change of approximately 200 C. was required to return thefilm essentially to the antiferromagnetic state. Repeated temperaturecycling of the film between C. and C. indicated the transition to bestable.

The thermal hysteresis loop illustrated in FIG. 3 and produced by aniron-rhodium film treated in accordance with the double-annealtechniques of this invention is characterized by a first ordertransition from the antiferromagnetic state to the ferromagnetic state(as illustrated by a measured magnetization of approximately 1 l5emu/gm. in a 1000 oersted field when heated above 100 C.). Uponsubsequent cooling of the film below 60 C. the measured magnetization ofthe film (and therefore the percentage of the film in the ferromagneticstate) remains substantially constant to a temperature of approximately50 C. whereafter the film returns to an essentially antiferromagneticstate of 8 emu/gm. (:2 emu/gm. error in the measured film magnetization)in a first order transition. Thus approximately 95 percent of the filmundergoes transition between the ferromagnetic and antiferromagneticstates after annealing a conventional 35 percent transformed film havinga broad hysteresis in an atmosphere having an oxygen concentrationgreater than 10 parts per million oxygen. Another significantcharacteristic in iron-rhodium films ofthis invention is the broadthermal hysteresis of the film, i.e. at the mean magnetization of thefilm i.e. approximately 60 emu/gm, the film is characterized by athermal hysteresis of approximately 80 C. Although repeated cycling ofthe film through the thermal hysteresis loop of the film produces someslight diminishment in the thermal hysteresis, iron-rhodium films afterl0 thermal cycles between -l95 C. and 100 C. still have been found toretain essentially the original thermal hysteresis.

The iron-rhodium film of this invention also is characterized by a rapidtransition to the ferromagnetic state upon heating above the criticaltemperature of the film. As can be seen from FIG. 3, the magnetizationof the film in a l,000 oersted field increases from a value ofapproximately 8 emu/gm. to a maximum value of approximately 115 emu/gm.between 20 C. and 90 C., e.g. over 90 percent of the film is transformedbetween the antiferromagnetic and ferromagnetic states within atemperature span of 70 C.

In general, the magnetic hysteresis loop of the iron-rhodium film ofthis invention is relatively square exhibiting a remanent magnetizationto saturation magnetization ratio of approximately 0.7. A sample 550Athick iron-rhodium film given a double-anneal treatment in accordancewith this invention had a coercive force of approximately oersteds.

Transmission electromicrographs were taken of iron-rhodium films formedunder identical conditions except for the oxidation anneal. One film wasannealed in vacuum and produced an incomplete transition characterizedby the hysteresis curve of H6. 2 while the second film was annealed in aflowing nitrogen atmosphere containing oxygen in concentrations between10 and 200 parts per million and produced a complete transitioncharacterized by the hysteresis curve of FIG. 3. Most aspects of themicrostructure, eg. grain size, stacking faults, twins, etc., werequalitatively identi cal although the film with the complete transitionshowed a much more mottled structure within the grains than did the filmhaving the incomplete transition. Electron beam microscopic examinationindicates that the mottling arises from a discrete array of particlesvery regularly arranged in a two dimensional square network. The networkappears to be uniform within a grain and has a periodicity ofapproximately 100 A. It may be postulated that the dirty appearance is afine dispersion of an oxide phase.

The films of this invention are particularly adapted for utilization asa digital information recording medium 19 such as is shown in FIG.wherein an iron-rhodium film 20 less than 1 mil thick and having atransition characteristic similar to that illustrated in FIG. 3 issituated atop a thermally conductive substrate 22 of a material such assilicon or quartz. The substrate is secured to thermoelectric base 24,e.g. bismuth telluride, lead telluride, antimony telluride, silverindium telluride, copper gallium telluride, etc. by a suitable adhesive,e.g. solder layer 26, to permit temperature cycling of the iron-rhodiumfilm by the thermoelectric base through substrate 22 upon electricalenergization of the thermoelectric base from a DC source (not shown)through leads 28 and 30. Because iron-rhodium film 20 is thermallyswitched between the ferromagnetic and antiferromagnetic states in afirst order transition producing a variation in film volume, adjacentrecording media 19A and 19B forming a memory unit are spaced by asuitable span, e. g. 2 percent of the film dimension, to permitnondestructive thermal expansion. When the dimensions of the recordingmedium are sufficiently small however, the memory unit desirably isformed as a unitary structure to avoid isolation of information storagesites.

To record information in selected sites of 1 mil diameter or less alongthe iron-rhodium film thermoelectric layer 24 initially is energizedwith DC current in a first direction to cool the structure below thetemperature, t, or approximately l50 C. in the hysteresis loop of FIG.3, at which the film becomes essentially antiferromagnetic therebyerasing any residual magnetism in the film. Electrical energization ofthermoelectric base 24 than is terminated to permit the temperature ofthe iron-rhodium film to increase to a biasing level, T,, orapproximately 20 C., whereat the film remains in an antiferromagneticstate below the critical temperature producing a first order transitionof the iron-rhodium film to a ferromagnetic state. An electron beam froman addressable electron gun, such as is described in US. Pat. b. Ser.No. 671,353, and now US. Pat. No. 3,491,236 filed Sept. 28, 1967, in thename of Sterling Newberry and assigned to the assignee of the presentinvention, then is irradiated upon selected bit sites 20A of theiron-rhodium film to heat the irradiated bit sites above the transitiontemperature of the film, e.g. above 120 C., and the irradiated bit sitesare transformed to the ferromagnetic state in a first order transition.Upon removal of the electron beam, the irradiated bit sites, i.e. 20A,cool to the biasing temperature, T,. After the application of asufficiently large magnetic field for a short time, e.g. a pulsed fieldgreater than 300 oersteds, the irradiated bit sites possess aferromagnetism indicative of information of a first magnitude. Those bitsites 20B not irradiated during recording remain in theantiferromagnetic state thereby storing digital information of adiffering magnitude. Thus, the selectively recorded ironrhodium film isof homogeneous composition and characterized by a plurality of bit sitesin either a magnetic or a nonmagnetic state dependent upon the magnitudeof information recorded at the individual bit sites. In general, an 8RV, 2X ampere electron beam irradiation of a 10 micron diameter regionof an iron-rhodium film for 4 milliseconds has been found adequate toconvert the irradiated bit sites from the antiferromagnetic state to theferromagnetic state. Adjacent bit sites were not raised above thecritical transition temperature and remained essentiallyantiferromagnetic.

Readout of the recorded information from the iron-rhodium is achieved bythe application of a pulsed magnetic field in excess of 300 oersteds tothe film to align the domains in ferromagnetic bit sites A therebyproducing a film characterized by ferromagnetic bits with amagnetization in a given direction interspaced with essentiallyantiferromagnetic bit sites 208 having zero net measurablemagnetization. Because detection of the alignment direction of theindividual bit sites is not required, visual readout can be easilyeffected by coating the iron-rhodium film with a colloidal solution ofiron oxide particles (or Bitter solution) which particles drift to themagnetized bit sites in the recording medium. Thus a plurality ofobservable dark areas, e.g. spots 50 shown in FIG. 6, are produced atthe electron beam irradiated ferromagnetic bit sites with the drift ofiron particles from the essentially antiferromagnetic bit sites 20Bresulting in a relatively clear liquid coating at such sites. whenextremely high speed is desired for readout, other conventional methodsof magnetic detection, e.g. electron beam microscopy can be employed tolocated the magnetized bit sites. To erase the recorded information,thermoelectric base 24 is again energized to reduce the temperature ofiron-rhodium film 20 to T. whereupon the entire film returns to theantiferromagnetic state and the previously recorded information iserased.

Erasure of the recorded information from the iron-rhodium film also canbe effected mechanically by the application of a strain to the filmthereby returning the strained portion of the film to essentially theantiferromagnetic state, as is depicted by the curves of FIG. 7. Therethermal hysteresis curve was obtained by temperature cycling a doublyannealed ironrhodium film along hysteresis loop 33 to C. to transformthe film to the ferromagnetic state and subsequently cooling the film toapproximately 15 C. whereupon the magnetization of the film returnedalong the hysteresis loop to a value of approximately 66 emu/gm.(identified by reference numeral 34). The film then was hand rubbed witha cotton swab for less than 20 20 and the magnetization decreased (asshown by dotted line 35) to a value of approximately 9 emu/gm. with avariation of only 5 C. in the temperature of the iron-rhodium film.Continued rubbing of the iron-rhodium film with the cotton swab reducedthe measured magnetization of the film at 10 C. to less than 5 emu/gm.and low magnetization state obtained by the rubbing induced strainremained stable as the film was cooled to a temperature below C. Uponsubsequent heating the film to 140 C. however, the ferromagneticcharacteristic of the film returned as exemplified by solid hysteresisloop 36, although the thermal hysteresis curve is somewhat narrowed bythe effects of the strain upon the film. In general, the quantity ofstrain applied to the iron-rhodium film to effect an erasure of therecorded information should be greater than 0.3 percent but not so greatas to cause the body centered cubic structure to transform to theparamagnetic face centered cubic structure.

Desirably, a small quantity of palladium is added to the ironrhodiumfilm to shift the critical temperature of the film to the magnetic stateto approximately 30 C., e.g. 5 C. above room temperature to reduce theelectron beam power required to transform bit sites to the ferromagneticstate while retaining the nonvolatile characteristics of the medium.Similarly cooling apparatus, such as thermoelectric base 24, can beomitted when sufficient iridium or platinum is introduced into theiron-rhodium film to shift the hysteresis loop by an amount positioningT at 25 C. The film then can be raised to the biasing temperature T,, atthe threshold of the critical film transition temperature by currentthrough the film or by electron beam impingement upon the entire filmplane. Information is recorded at selected bit sites by heating with asecond electron beam to increase the temperature of the irradiated sitesabove the critical transition temperature of the film thereby convertingthe irradiated sites to the ferromagnetic state. Similarly, otherconventional heat sources, e.g. visible or infrared light, can beemployed to raise an iron-rhodium-iridium film to the biasingtemperature of the film.

Although alignment of the magnetic domains within the fer romagnetic bitsites of iron-rhodium film 20 has been described as being produced bythe application of a magnetic field to the film after the selectivelyheated film has been cooled to a biasing temperature, T the magneticfield also can be applied to the film simultaneously with the selectiveheating of the film. In such event, selective heating of the film bitsites utilizing a laser beam is preferred to inhibit undersireddeflection of the beam by the field magnetizing the bit sites convertedto the ferromagnetic state. Thus, to write information the film iscooled to a temperature 'I whereat the entire film is converted to theantiferromagnetic state and the entire film is thereupon heated to abiasing temperature T,, at the threshold of a first order transition tothe ferromagnetic state. A laser bean then is selectively impinged uponindividual bit sites of the film to raise the bit sites above thetransition temperature converting the bit sites to the ferromagneticstate in a first order transition. Upon removal of the writing laserbeam from each irradiated bit site the irradiated sites return to thebiasing temperature along the thermal hysteresis loop and remain in aferromagnetic state relative to the unheated bit sites.

The magnitude of the magnetic field required to magnetize aferromagnetic, but demagnetized, iron-rhodium film is depicted by thecurves of FIG. 8 wherein curves 37 and 38 represent the saturationmagnetization and remanent magnetization of the film, respectively. Ascan be seen from curve 37, an applied field of 800 oersteds is requiredfor a ferromagnetic iron-rhodium film at room temperature to reach 0.9of the maximum saturation magnetization of the film. Upon termination ofthe applied magnetic field to the film, the magnetism of the filmdecreases to a remanent value, identified by curve 38 of FIG. 8,approximately 60 emu/gm. below the saturation magnetization obtainablewith the given field.

Another advantageous attribute of iron-rhodium films in accordance withthis invention is the adjustable saturation flux density of the film ascan be observed from FIG. 9 wherein a major thermal hysteresis loop 40of an iron-rhodium film in a L000 oersted field is depicted. Thus, ifthe iron-rhodium film upon temperature cycling is cooled only partiallyin the return cycle, e.g. interrupted at l C. along hysteresis loop 40,and then reheated, the saturation magnetization of the film, as depictedby curve 42 remains substantially constant at 60 emu/gm. over atemperature span between -l0 C. and 30 C. When the cooling cycle of themajor hysteresis loop is interrupted at a more reduced temperature, e.g.58 C. a constant saturation magnetization of 23 emu/gm, as portrayed bycurve 44, is maintained when the temperature of the film is cycledbetween 58 C. and =30 C. In general, the allowable temperature excursionpermissible without change in film magnetization is somewhat less thanthe thermal span between the temperature at which the cooling cycle ofthe film is interrupted and the critical transition temperature ofthefilm to the ferromagnetic state, curve 48. Thus the magnetization of theiron-rhodium film (indicative of the saturation flux density of thefilm) can be adjusted merely by an interruption of the cooling cycle ofthe film at a desired thermal location subsequent to the transitionofthe film to the ferromagnetic state. Similarly, alterations in thesaturation flux density of the ironrhodium film can be effected duringthe heating cycle of the major hysteresis loop by interrupting theheating cycle prior to the maximum magnetization of the film, e.g. at 55c., and cooling the film along minor hysteresis loop 46 to atemperature, T producing the desired flux density, as indicated by themeasured magnetization of the film in the 1,000 oersted field.Subsequent reheating and cooling of the film within a temperature spanbetween T, and the critical transition temperature Tp of the minorhysteresis loop produces a negligible variation in the saturation fluxdensity from the present value. Thus the flux of the iron-rhodium filmcan be adjusted to a desired value namely by an alteration in thethermal cycling of the film.

The iron-rhodium film of this invention also can be suitably employed asa temperature sensor to indicate the maximum excursions at thetermination of a temperature cycle. For example, when a material iscooled from a temperature above 150 C., the magnetization of aniron-rhodium film contacting the material is indicative of the minimumtemperature reached during the cooling cycle notwithstanding a slightincrease in iron-rhodium film temperature upon removal from contact withthe material. Because the thermal sensitivity of the iron-rhodium filmis dependent upon the disposition of the thermal hysteresis loop of thefilm relative to the abscissa, the slope of the iron-rhodium hysteresiscurve preferably is angu larly disposed relative to the temperaturescale over the temperature range of interest. To obtain measurementsover a broad temperature range, the oxidation anneal preferably isconducted at the upper limits of the allowed oxygen concentration toreduce the squareness of the thermal hysteresis loop while highlyprecise temperature measurements over a narrow range best is effected bya relatively square thermal hysteresis loop exhibiting a rapid change inmagnetization over the temperature range of interest.

Although the method of recording employing the first order transition offilm bit sites between the magnetic and nonmag netic states has beendescribed herein by specific reference to iron-rhodium alloy films, anymaterial characterized by a first order transition from the magnetic tothe nonmagnetic states with an associated thermal hysteresis, e.g.manganese bismuth, manganese arsenide or chromium manganese antimonide,also can be employed for recording in accordance with the techniques ofthis invention. The technique of this invention however is to bedistinguished from the use of manganese bismuth in conventional Curiepoint writing because of the reliance of the herewith disclosed wiringtechnique on the thermal hysteresis loop of the material, e.g. with abiasing temperature along the thermal hysteresis loop, while Curie PointWriting with manganese bismuth ignores the thermal hysteresis of themanganese bismuth and temperature cycles completely through the thermalhysteresis loop. Curie point writing with manganese bismuth then recordsinformation by the alignment of the magnetization at. the various bitsites.

Iclaim:

l. A film ofiron-rhodium and alloys thereof comprising:

alternately deposited layers of iron and rhodium. annealed together, andtotaling less than 1 mil in thickness;

said film composed of from 50 to 65 atom percent rhodium;

said annealed layers characterized by a first order transition betweenferromagnetic and antiferromagnetic states in excess of 50 percent ofthe film when the film is temperature cycled through the thermalhysteresis loop of the film.

2. A film of iron-rhodium according to claim 1 wherein said film isfurther characterized by a thermal hysteresis loop having a thermalwidth between 10 C. and 200 C. at the mean magnetization of said film.

3. A film of iron-rhodium according to claim I characterized by thetransformation of at least percent of the film from theantiferromagnetic state to the ferromagnetic state upon heating the filmto a temperature 70 C. above the critical transition temperature of thefilm.

4. A film of iron-rhodium according to claim 1 wherein said film is lessthan 3,000A thick and exhibits a first order transition between theferromagnetic and antiferromagnetic states in excess of 90 percent ofthe film when temperature cycled through the thermal hysteresis loop ofthe film.

5. A film of iron-rhodium and alloys thereof according to claim 4wherein said film includes less than l0 percent of a metal, codepositedwith at least one of said iron and rhodium layers, selected from thegroup consisting of ruthenium, osmiurn, iridium, and platinum.

6. A film of iron-rhodium and alloys thereof according to claim 4wherein said film contains less than 10 atom percent of a metal,codeposited with at least one of said iron and rhodium layers, selectedfrom the group consisting of palladi um, vanadium, manganese and gold.

7. A film of iron-rhodium and alloys thereof according to claim 4,wherein said film includes less than 10 atom percent of a metal,deposited as a layer within at least one set of said iron and rhodiumlayers, selected from the group consisting of ruthenium, osmium, iridiumand platinum.

and rhodium layers, selected from the group consisting of palladium,Vanadium, manganese and platinum.

2. A film of iron-rhodium according to claim 1 wherein said film isfurther characterized by a thermal hysteresis loop having a thermalwidth between 10* C. and 200* C. at the mean magnetization of said film.3. A film of iron-rhodium according to claim 1 characterized by thetransformation of at least 90 percent of the film from theantiferromagnetic state to the ferromagnetic state upon heating the filmto a temperature 70* C. above the critical transition temperature of thefilm.
 4. A film of iron-rhodium according to claim 1 wherein said filmis less than 3,000A thick and exhibits a first order transition betweenthe ferromagnetic and antiferromagnetic states in excess of 90 percentof the film when temperature cycled through the thermal hysteresis loopof the film.
 5. A film of iron-rhodium and alloys thereof according toclaim 4 wherein said film includes less than 10 percent of a metal,codeposited with at least one of said iron and rhodium layers, selectedfrom the group consisting of ruthenium, osmium, iridium, and platinum.6. A film of iron-rhodium and alloys thereof according to claim 4wherein said film contains less than 10 atom percent of a metal,codeposited with at least one of said iron and rhodium layers, selectedfrom the group consisting of palladium, vanadium, manganese and gold. 7.A film of iron-rhodium and alloys thereof according to claim 4, whereinsaid film includes less than 10 atom percent of a metal, deposited as alayer within at least one set of said iron and rhodium layers, selectedfrom the group consisting of ruthenium, osmium, iridium and platinum. 8.A film of iron-rhodium and alloys thereof according to claim 4 whereinsaid film contains less that 10 atom percent of a metal, deposited as alayer within at least one set of said iron and rhodium layers, selectedfrom the group consisting of palladium, Vanadium, manganese andplatinum.